In this specification the term “light” will be used in the sense that it is used in optical systems to mean not just visible light but also electromagnetic radiation having a wavelength between 1000 nanometres (nm) and 3000 nm.
Single wavelength lasers are important for a number of applications in optical telecommunications and signal processing applications. These include multiple channel optical telecommunications networks using wavelength division multiplexing (WDM). Such networks can provide advanced features, such as wavelength routing, wavelength conversion, adding and dropping of channels and wavelength manipulation in much the same way as in time slot manipulation in time division multiplexed systems. Many of these systems operate in the C-band in the range 1530 to 1570 nm.
Tuneable lasers for use in such optical communications systems, particularly in connection with the WDM telecommunication systems, are known. A known tuneable system comprises stacks of single wavelength distributed Bragg reflectors (DBR) lasers, which can be individually selected, or tuned over a narrow range, or by a wide tuning range tuneable laser that can be electronically driven to provide the wavelength required. Limited tuning range tuneable lasers that rely upon thermal effects for tuning are also known.
U.S. Pat. No. 4,896,325 discloses a wavelength tuneable laser having sampled gratings at the front and rear of its gain region. The laser described in that specification relies on the use of two different gratings which produce slightly different reflection combs in the front and rear gratings. These provide feedback into the device. The gratings can be current tuned in wavelength with respect to each other. Co-incidence of a maximum from each of the front and rear gratings is referred to as a supermode. To switch the device between supermodes requires a small incremental electrical current into one of the gratings to cause a different pair of maxima to coincide in the manner of a vernier. By applying electrical currents to the two gratings so that the corresponding maxima track, continuous tuning within a supermode can be achieved.
In summary, for a given set of drive currents in the front and rear grating sections, there can only be a simultaneous correspondence in reflection peak at one wavelength, as a consequence of which the device lases at that wavelength. To change that wavelength a different current is applied to the front and rear gratings. Thus the front and rear gratings operate in a vernier mode, in which the wavelengths of correspondence determine a supermode wavelength.
An improved form of selective grating is the phase shifted grating, as described in UK-A-2 337 135, the contents of which are incorporated herein by way of reference.
The term “phase shifted grating” as used herein is used to describe and define a selectively reflecting distributed Bragg grating which is constructed and operates in the manner described and claimed in UK-A-2 337 135. In other words, a phase shifted grating is one in which the grating structure comprises a plurality of repeat gratings in which each grating unit comprises a series of adjacent diffraction gratings having the same pitch, and is characterised in that the grating units and adjacent gratings within a grating unit are separated by a phase shift of substantially pi (π) radians and in which at least two of the gratings within a grating unit have different lengths, the lengths being selected so as to provide a predetermined reflection spectrum.
Details on the construction and operation of the phase shifted-grating are to be found in UK-A-2 337 135. The Bragg gratings may be defined using e-beam writing onto suitable photoresist as a processing step.